Colorectal cancer is the second leading cause of death due to cancer in the United States. The mortality rate of this disease is high principally because of the advanced stage of the disease at the time of diagnosis, or the recurrence of a previously treated cancer. The liver is a major target organ for colonization of colon cancer cells. The long term objective of this proposal is to develop novel immunological approaches that are effective at inhibiting the growth of human colon cancer metastatic to the liver. The studies will focus on the use of a murine-human chimeric SF-25 monoclonal antibody (c-SF-25 Mab) that recognizes a cell surface antigen highly expressed in primary as well as metastatic human colon cancers. This construct induced antibody-dependent cell-mediated cytotoxicity in vitro and was capable of targeting human effector cells to hepatic metastasis of colon cancer in vivo. More importantly, c-SF-25 Mab appears to activate human lymphocytes to promote apoptosis through non-Fas-mediated mechanisms of non-adjacent bystander tumor cells by soluble factors. To establish an experimental immunotherapy based on this novel antitumor mechanism, the proposed studies will be directed to define soluble factors responsible for tumor cell killing and obtain sequence information from purification of these factors. In conjunction with these studies, an effort will be made to determine the protein sequence of purified SF-25 antigen and clone a complementary DNA (cDNA) encoding this tumor-associated antigen. To accomplish these goals, the following experiments are planned: 1) To investigate the mechanisms of non-Fas-mediated apoptosis and bystander lysis of colon cancer cells, and determine what soluble factors mediate this bystander effect using various Fas-resistant target cells and neutralizing polyclonal antibodies. It also will be determined if apoptosis is mediated through known members of the tumor necrosis factor receptor family and the activation of ICE family proteases using recombinant cytokine receptors and tetrapeptide protease inhibitors, respectively. The in vivo studies will be performed to determine if c-SF-25 Mab activates human effector cells locally in the tumor microenvironment of liver metastases of human colon cancers. 2) To purify apoptosis-promoting factors and clone the cDNA encoding these soluble factors using olignucleotide probes derived from protein microsequencing information. 3) To isolate SF-25 antigen and clone cDNA using protein sequence information of purified antigen. Collectively, these experiments should provide a foundation for new immunotherapeutic approaches that may eventually be translated into innovative clinical treatment regimens of human colon cancer metastatic to the liver, otherwise untreatable by conventional therapies.